1. Field of the Invention
The present invention relates to a pressure cooker system, which cooks food under high pressure, the pressure cooker being constructed so that an electric power supply to a heating device is automatically controlled using a sensor.
2. Description of the Prior Art
There are pressure cookers in which the internal pressure from vapor generated from the food is raised and the boiling point is increased by tightly covering the food so that the heating time can be shortened.
Such pressure cookers have the following three heating modes. One is a standby mode, which is effective until a pressure control valve begins to operate shortly after the internal pressure of the pressure cooker increases due to a gradual rise in the temperature of the food after heating begins; the second is a pressure mode, which maintains a specific internal pressure while the pressure control valve is operating; and the third is an escape mode, which stops an electric power supply to the heating device and waits for the internal pressure to drop close to atmospheric pressure.
Of these heating modes, the most important heating mode having the greatest effect on the heating condition of the food is the pressure mode, which maintains a specific internal pressure. Heating times which are set forth in cook books that include with pressure cooking and the like for household use, are generally the times required for this pressure mode. Because the required times are short, failure may easily be caused if the times are not managed accurately.
However, since the standby time required for the standby mode differs depending upon factors such as the quantity of food and the calories of the heating device, a person operating the pressure cooker must constantly attend the cooker until the pressure control valve begins to operate, and must then start a timer into which the heating time is set, after confirming the point at which this operation began.
It is generally not necessary to operate the heating device at full power after the pressure mode has been reached. This avoids not only wasteful energy consumption, but also to cause the food to be unnecessarily agitated. Accordingly, when the pressure control valve begins to operate, it is also necessary for the person operating the pressure cooker to reduce the heat of the heating device to the minimum power at which the pressure control valve will operate.
To eliminate this kind of troublesome operation, there have been electric pressure cookers, the operability of which is improved by the use of a temperature sensor to detect the temperature of the cooker.
A control apparatus for electric pressure cooker as set forth in Japanese Laid-Open Patent Publication No. 63-9846, provides a thermal sensor joined to the bottom of a pressure cavity, and a thermal detecting device which outputs various control signals based upon signals from this device. When it is detected that the pressure cavity has attained a certain control temperature, a time switch operates, and electric power is reduced by turning the electricity to the heater off and on.
In other words, the heating time required for the pressure mode is preset with the time switch, and full power is applied during the standby mode. When the pressure cavity reaches a specified temperature, the time switch starts and the power is also automatically reduced. This means that the switching operation from the standby mode to the pressure mode is automatically performed by the control apparatus without constant attendance to the cooker.
A pressure cooker as set forth in Japanese Laid-Open Patent Publication No. 57-10264 is provided with a thermal detecting device which detects a drop in the internal pressure of a pressure cavity to atmospheric pressure, and it also performs a reporting by means of output from this device.
In other words, completion of the escape mode is detected by the temperature of the pressure cavity, and this is reported.
However, in either of the conventional cookers mentioned above, a thermal sensor detects the temperature of the pressure cavity's exterior surface, indirectly inferring the internal temperature, and therefore it is impossible to directly and accurately control the internal temperature of the cavity.
Since the heat conduction from the inside of the pot to the thermal sensor is delayed due to the thickness of the pot, the internal temperature exceeds the temperature to be controlled, and overshooting occurs. This kind of overshooting occurs to a rather large extent, particularly when the quantity of food is small, because the standby period is short.
Moreover, when foods like soup adheres to the contact area between the thermal sensor and the pot, the heat conduction becomes more difficult and the internal temperature increases, that is, the pressure rises. Hence, cooking does not proceed properly and there is an increase in danger.
Moreover, the operator must manually set the heating time using a timer while referring to the cook book etc. Although the timing operated by the timer is automatically controlled, heating is basically not automatic.